Recently, lithium secondary batteries using non-aqueous electrolyte are increasingly used as a power source for portable electronic devices due to high voltage, high capacity, high output and low weight. However, such lithium secondary batteries have a safety problem and thus attempts to solve this problem are being made. When lithium secondary battery is overcharged, excess lithium flows out from a positive electrode and is inserted into a negative electrode, while very highly reactive lithium metal is deposited on the negative electrode surface, and the positive electrode becomes thermally unstable. This results in rapid exothermic reactions due to the decomposition reaction of an organic solvent used as electrolyte, thus causing safety problems, such as battery fire and explosion.
Furthermore, when conductive materials, such as nails, penetrate the battery, the electrochemical energy within the battery is converted into thermal energy while generating heat rapidly. The generated heat causes rapid exothermic reactions by the chemical reaction of the positive or negative electrode materials, resulting in safety problems, such as battery fire and explosion.
Moreover, the nail penetration, compression, impact and high temperature exposure of the battery lead to a local short circuit within the positive and negative electrodes of the battery. At this time, excessive currents locally flow to generate heat. As the magnitude of a short circuit current caused by the local short circuit is inversely proportional to resistance, the short circuit current flows toward portions with low resistance, mainly through a metal foil used as a current collector. The calculation of heat generation in this case indicates that a very high heat generation locally occurs centering a part into which a nail penetrated, as described in FIG. 1.
If heat generation occurs within the battery, the positive and negative electrodes and the electrolyte included in the battery will either react with each other or combust, and eventually the battery will catch fire or explode, since this reaction is a very high exothermic reaction. For this reason, care is required to make sure that rapid heat generation within the battery does not occur.
If the battery is pressed with a heavy object, subjected to strong impact or exposed to high temperature, such a safety problem will also occur. This safety problem will be more serious, as the capacity of lithium secondary batteries increases, leading to an increase in energy density.
Generally, lithium secondary batteries use a lithium-containing transition metal oxide as a positive active material, which is one or more selected from the group consisting of, for example, LiCoO2, LiNiO2, LiMn2O4, LiMnO2 and LiNi1-XCoXO2 (0<X<1). As a negative active material, carbon, lithium metal or alloy is used, and other metal oxides, such as TiO2 and SnO2, may also be used which can intercalate and deintercalate lithium and have a potential of less than 2V for lithium. Furthermore, as a non-aqueous electrolyte, cyclic and linear carbonates are used. The non-aqueous electrolyte contains a lithium salt selected from the group consisting of, for example LiClO4, LiCF3SO3, LiPF6, LiBF4, LiAsF6, and LiN(CF3SO2)2.
In the lithium secondary battery fabricated as such, the positive or negative electrode and the non-aqueous electrolyte can react with each other at high temperature, particularly in a charged condition, thus causing high reaction heat. A series of exothermic reactions resulting from this heat cause the safety problem.
Although the safety problem in an overcharged state can be solved by the addition of additives to the non-aqueous electrolyte, the battery safety in the above-mentioned conditions, such as nail penetration, pressing, impact and exposure to high temperature, cannot be secured by the addition of additives to the non-aqueous electrolyte.